How Doctors Chart a Premature Baby's Growth — and How That Chart Has Improved
Understanding the Fenton growth charts of 2003, 2013, and 2025
The growth chart in the corner of the incubator
If your baby is in a neonatal intensive care unit, you have probably seen a curved chart on a screen or on the wall, with a dot marking your baby's weight each day. That chart answers a deceptively simple question: is this baby growing the way we would hope? For a baby born early, the answer is harder than it sounds, because the goal is not to grow like a newborn at home — it is to grow the way the baby would have grown if still inside the womb.
The chart most units use to make that comparison is called the Fenton growth chart, named after the researcher Tanis Fenton. It has been redrawn three times, in 2003 [1], 2013 [2], and 2025 [3]. Each version aimed to make the chart a more honest picture of healthy growth. This article walks through what changed and why it matters for your baby.
Why a special chart is needed at all
A baby grows astonishingly fast in the womb. Between about 22 weeks of pregnancy and full term, a fetus multiplies its weight more than tenfold — from a few hundred grams to several kilograms [2]. A baby born at 26 weeks is meant to keep growing at roughly that rapid pace outside the womb. But there is no easy way to weigh a fetus directly, so researchers estimate how a fetus grows by measuring thousands of babies at the moment they are born at each gestational age, and stitching those measurements into a curve.
To put real numbers on that astonishing pace, the 2013 data show a girl's middle-of-the-range weight rising from about 520 grams at 22 weeks to about 5,360 grams by ten weeks after term [2] — roughly the difference between a small apple and a healthy newborn.
This was first done in a widely used way in 1976, with a chart by two doctors named Babson and Benda [4]. It was a real advance for its time, but it was based on relatively few babies, it only started at 26 weeks, and its lines were spaced coarsely [1]. As more and better data became available, it needed updating — and that is where the Fenton charts come in.
2003: a bigger, sharper picture
The first Fenton chart [1] gathered together many newer studies of baby measurements and combined them into a single, sharper chart. Three things improved. It started earlier, at 22 weeks, so even the tiniest, earliest babies could be plotted. Its lines were finer, so a baby's position could be read more precisely. And the researchers checked the new chart against real growth records from a large American research network to make sure it matched reality [1].
One striking finding was that the old 1976 chart had been a little too generous at the bottom of the range. A baby that looked "about average-low" on the old chart often sat closer to the genuinely small end on the new one [1]. In other words, the better data revealed that some small babies had been quietly under-recognised.
The 2003 chart had one notable limitation: it used a single chart for boys and girls together, even though boys and girls grow at slightly different rates.
2013: separate charts for boys and girls, and a smoother handover
The second Fenton chart [2] is the one most hospitals still use today, and it fixed several things at once. It created separate charts for boys and girls, so each baby is compared with others of the same sex. It was carefully matched up with the World Health Organization's growth standard — the chart used for healthy children worldwide — so that when a premature baby reaches the age of a full-term newborn, the line continues smoothly onto the standard children's chart without a sudden jump [2]. And it was redrawn so that a baby could be plotted at their exact age: a baby who is 25 weeks and 3 days is marked between the 25- and 26-week lines, rather than rounded down to a whole week [2].
These curves were built from an enormous amount of information — almost four million births from six countries (Germany, the United States, Italy, Australia, Scotland, and Canada), including tens of thousands of very early babies [2]. Among the studies it drew on were large American measurements [5]. For most babies, the 2003 and 2013 charts give very similar answers; the real change came with the newest version.
2025: a picture of how a healthy baby should grow
The newest Fenton chart [3] makes the most important change of all, and it is worth understanding because it may affect what you are told about your baby.
Here is the subtle problem the researchers set out to fix. Many babies are born very early because something had slowed their growth in the womb. If you build a growth chart from all early births, you end up including a lot of babies who were not growing well — which drags the "normal" line downward. The chart then describes how premature babies typically are, rather than how a healthy baby should be.
The 2025 chart deliberately leaves out babies whose growth in the womb appeared to be affected, using around 4.8 million births from fifteen countries to draw a cleaner picture of healthy growth [3]. Because the unhealthy, smaller babies are no longer pulling the line down, the new "normal" curve sits a little higher than before, especially for the earliest babies.
What does that mean in practice? A baby of exactly the same weight will appear slightly smaller on the new chart than on the old one. As a result, more babies are now identified as "small for gestational age" — meaning smaller than expected for their stage — by as much as 29% of babies at 28 weeks [3]. This difference is largest in the early weeks and fades as a baby approaches full-term age.
What a "small for gestational age" label really means
If your baby is newly labelled small for gestational age on the 2025 chart, it is natural to feel worried. It helps to know what the label is for. It is not a diagnosis of illness. It is a flag that tells the care team to watch your baby's growth more closely and to make sure nutrition is well matched to your baby's needs. Many babies who are small early on grow well with good care.
It also helps to know that doctors have long understood that which chart you use changes the count. Studies comparing the Fenton charts with another well-known set of charts, called INTERGROWTH-21st, have found that the two disagree about which babies are small [6]. That is exactly why the care team aims to use one chart consistently for your baby, rather than switching back and forth.
How the chart actually works behind the scenes
You may notice the team reading off a "percentile" or a "Z-score." A percentile simply means where your baby sits among 100 similar babies — being on the 30th percentile for weight means about 30 of 100 babies that age would weigh less. A Z-score is a more precise version of the same idea. These numbers are produced automatically by free online calculators that hospitals use, which can switch between the 2003, 2013, and 2025 Fenton charts [7]. The mathematics that lets a computer turn a single weight into an exact percentile was developed decades ago and is used in growth charts the world over [8], including the World Health Organization's own standard for healthy children [9].
What researchers are working on next
The big idea running through the 2025 update — building a chart from healthy growth rather than from all growth — is part of a wider shift in newborn medicine. Another major international project, INTERGROWTH-21st, took the same idea in a different way, following healthy premature babies over time to build its own standard, and described this as a genuine change in thinking about how premature growth should be judged [10]. Researchers are now studying which of these approaches best predicts how children do later in life, and how to use the new, stricter charts to support good growth without overfeeding babies [11].
The takeaway for families
Over two decades, the Fenton growth chart has moved from a single combined chart to separate boy and girl charts, then to a chart that more faithfully reflects how a healthy baby grows in the womb. If your unit uses the newest version, your baby may be called small for gestational age when they might not have been before — but that change reflects a better yardstick, not a worse baby. The chart is a tool to help your care team give your baby exactly the attention and nutrition they need, and the questions you ask about that dot on the screen are always worth raising with the team caring for your child.
References
- Fenton TR. A new growth chart for preterm babies: Babson and Benda's chart updated with recent data and a new format. BMC Pediatrics. 2003;3:13. doi:10.1186/1471-2431-3-13 ↩
- Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatrics. 2013;13:59. doi:10.1186/1471-2431-13-59 ↩
- Fenton TR, Elmrayed S, Alshaikh BN. Fenton third-generation growth charts of preterm infants without abnormal fetal growth: a systematic review and meta-analysis. Paediatric and Perinatal Epidemiology. 2025. doi:10.1111/ppe.70035 ↩
- Babson SG, Benda GI. Growth graphs for the clinical assessment of infants of varying gestational age. Journal of Pediatrics. 1976;89(5):814-820. doi:10.1016/S0022-3476(76)80815-380815-3) ↩
- Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New intrauterine growth curves based on United States data. Pediatrics. 2010;125(2):e214-e224. doi:10.1542/peds.2009-0913 ↩
- Kim YJ, Shin SH, Cho H, et al. Extrauterine growth restriction in extremely preterm infants based on the INTERGROWTH-21st Project Preterm Postnatal Follow-up Study growth charts and the Fenton growth charts. European Journal of Pediatrics. 2021;180(3):817-824. doi:10.1007/s00431-020-03796-0 ↩
- Chou JH, Roumiantsev S, Singh R. PediTools electronic growth chart calculators: applications in clinical care, research, and quality improvement. Journal of Medical Internet Research. 2020;22(1):e16204. doi:10.2196/16204 ↩
- Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Statistics in Medicine. 1992;11(10):1305-1319. doi:10.1002/sim.4780111005 ↩
- Borghi E, de Onis M, Garza C, et al. Construction of the World Health Organization child growth standards: selection of methods for attained growth curves. Statistics in Medicine. 2006;25(2):247-265. doi:10.1002/sim.2227 ↩
- Villar J, Giuliani F, Barros F, et al. Monitoring the postnatal growth of preterm infants: a paradigm change. Pediatrics. 2018;141(2):e20172467. doi:10.1542/peds.2017-2467 ↩
- Villar J, Giuliani F, Bhutta ZA, et al. Postnatal growth standards for preterm infants: the Preterm Postnatal Follow-up Study of the INTERGROWTH-21st Project. The Lancet Global Health. 2015;3(11):e681-e691. doi:10.1016/S2214-109X(15)00163-100163-1) ↩